1. Field of the Invention
This invention relates to scintigraphic imaging agents and reagents, and methods for producing such agents and reagents. Specifically, the invention relates to reagents that can be radiolabeled with technetium-99m (Tc-99m), methods and kits for making and radiolabeling such reagents, and methods for using such radiolabeled reagents to image sites of thrombus formation in a mammalian body.
2. Description of the Prior Art
Thrombosis and thromboembolism, in particular deep vein thrombosis (DVT) and pulmonary embolism (PE), are common clinical conditions that are associated with significant morbidity and mortality. It has been estimated that in the U.S. approximately 5 million patients experience one or more episodes of DVT per year and that over 500,000 cases of pulmonary embolism occur, resulting in 100,000 deaths (J. Seabold, Society of Nuclear Medicine Annual Meeting 1990). It has also been estimated that over 90% of all pulmonary emboli arise from DVT in the lower extremities. Anticoagulant therapy can effectively treat these conditions if applied early enough. However, such treatment is associated with risks (e.g. internal bleeding) that prevent unnecessary prophylactic application. More advanced techniques of thrombolytic intervention (such as the administration of recombinant tissue plasminogen activator or streptokinase) can be used in acute cases, but these techniques carry even greater risk. Moreover, effective clinical application of these techniques requires that the site of the offending thrombus be identified so as to monitor the effect of treatment.
For these reasons, a rapid means of localizing thrombi in vivo, most preferably using non-invasive methods, is highly desirable. Methods currently utilized for the identification of sites of deep-vein thrombosis are contrast venography and compression B-mode ultrasound; the choice of which technique is used depends on the expected location of the thrombus. However, the former technique is invasive and both techniques are uncomfortable for the patient. In addition, these methods are in many cases either unsuitable or yield inaccurate results.
Current methods used to diagnose PE include chest X-ray, electrocardiogram (EKG), areterial oxygen tension, perfusion and ventilation lung scans, and pulmonary angiography. Apart from the latter (invasive) procedure, none of these methods is capable of providing an unequivocal diagnosis.
In the field of nuclear medicine, certain pathological conditions are localized, or their extent is assessed, by detecting the distribution of small quantities of internally-administered radioactively labeled tracer compounds (called radiotracers or radiopharmaceuticals). Methods for detecting these radiopharmaceuticals are known generally as imaging or radioimaging methods.
A variety of radionuclides are known to be useful for radioimaging, including .sup.67 Ga, .sup.68 Ga, .sup.99m Tc (Tc-99m), .sup.111 In, .sup.123 I, .sup.125 I, .sup.169 Yb and .sup.186 Re. Of these radionuclides, Tc-99m and .sup.111 In are preferred single photon-emitting radionuclides and .sup.68 Ga is preferred as a positron-emitting radionuclide. Tc-99m is a preferred radionuclide because it emits gamma radiation at 140 keV, it has a physical half-life of 6 hours, and it is readily available on-site using a molybdenum-99/technetium-99m generator.
A gamma-emitting radiotracer that binds specifically to a component of a thrombus in preference to other tissues when administered in vivo can provide an external scintigraphic image which defines the location of the thrombus-bound radiotracer and hence the thrombus. Thrombi are constructs of blood cells, largely activated platelets, enmeshed in cross-linked fibrin. Activated platelets are particularly good targets for radioimaging thrombi because they are not normally found in circulating blood (which contains unactivated platelets).
Activated platelets express the GPIIb/IIIa receptor on their cell surfaces. The normal ligand for this receptor is fibrinogen (Plow et al., 1987, Perspectives in Inflammation, Neoplasia and Vascular Cell Biology, pp. 267-275). However, small, synthetic analogues, which may be but are not necessarily peptides, have been developed that bind to this receptor (examples include Klein et al., 1992, U.S. Pat. No. 5,086,069 and Egbertson et at., 1992, European Patent Application No. EPA 0478328A1). Although many of these synthetic molecules bind with only low affinity, others have been made that have very high affinity (see Egbertson et al., ibid.). This invention provides small, synthetic, radiolabeled (preferably Tc-99m, .sup.111 In or .sup.68 Ga labeled) compounds that bind to the GPIIb/IIIa receptor with high affinity, as scintigraphic agents for non-invasive imaging of thrombi in vivo.
Attempts to provide radiotracers for imaging thrombi are known in the prior art. These include autologous platelets, labeled with either .sup.111 In or .sup.99m Tc (Tc-99m), and .sup.123 I- and .sup.125 I-labeled fibrinogen (the latter detected with a gamma scintillation probe as opposed to a gamma camera). Additional radiolabeled compounds used to label thrombi include plasmin, plasminogen activators, heparin, fibronectin, fibrin Fragment E.sub.l and anti-fibrin and anti-platelet monoclonal antibodies [see Knight, 1990, Sero. Nucl. Med. 20:52-67 for review].
Compounds having the ability to bind to the platelet GPIIb/IIIa receptor are known in the prior art.
Ruoslahti & Pierschbacher, U.S. Pat. No. 4,578,079 describe peptides of sequence X-Arg-Gly-Asp-R-Y, wherein X and Y are either H or an amino acid, and R is Thr or Cys SEQ ID No:2, the peptides being capable of binding to platelets SEQ ID No:1.
Ruoslahti & Pierschbacher, U.S. Pat. No. 4,792,525 describe peptides of sequence Arg-Gly-Asp-X, wherein X is Ser, Thr or Cys, the peptides being capable of binding to platelets.
Klein et al., 1992, U.S. Pat. No. 5,086,069 disclose guanine derivatives that bind to the GPIIb/IIIa receptor.
Pierschbacher et al., 1989, PCT/US88/04403 disclose conformationally-restricted RGD-containing peptides for inhibiting cell attachment to a substratum.
Nutt et al., 1990, European Patent Application 90202015.5 disclose cyclic RGD peptides that are fibrinogen receptor antagonists.
Nutt et al., 1990, European Patent Application 90202030.4 disclose cyclic RGD peptides that are fibrinogen receptor antagonists.
Nutt et al., 1990, European Patent Application 90202031.2 disclose cyclic RGD peptides that are fibrinogen receptor antagonists.
Nutt et al., 1990, European Patent Application 90202032.0 disclose cyclic RGD peptides that are fibrinogen receptor antagonists.
Nutt et al., 1990, European Patent Application 90311148.2 disclose cyclic peptides that are fibrinogen receptor antagonists.
Nutt et al., 1990, European Patent Application 90311151.6 disclose cyclic peptides that are fibrinogen receptor antagonists.
Ali et al., 1990, European Patent Application 90311537.6 disclose cyclic peptides that are fibrinogen receptor antagonists.
Barker et al., 1991, PCT/US90/03788 disclose cyclic peptides for inhibiting platelet aggregation.
Pierschbacher et al., 1991, PCT/US91/02356 disclose cyclic peptides that are fibrinogen receptor antagonists.
Egbertson et al., 1992, European Patent Application 0478328A1 disclose tyrosine derivatives that bind with high affinity to the GPIIb/IIIa receptor.
Ojima et al., 1992, 204th Meeting, Amer. Chem. Soc. Abst. 44 disclose synthetic multimeric RDGF peptides useful in inhibiting platelet aggregation.
Hartman et al., 1992, J. Med. Chem. 35:4640-4642 describe tyrosine derivatives that have a high affinity for the GPIIb/IIIa receptor.
Radiolabeled peptides for radioimaging thrombi have been reported in the prior art.
Stuttle, 1990, PCT/GB90/00933 discloses radioactively labeled peptides containing from 3 to 10 amino acids comprising the sequence arginine-glycine-aspartic acid (RGD), capable of binding to an RGD binding site in vivo.
Rodwell et al., 1991, PCT/US91/03116 disclose conjugates of "molecular recognition units" with "effector domains".
The use of chelating agents for radiolabeling polypeptides, and methods for labeling peptides and polypeptides with Tc-99m are known in the prior art and are disclosed in co-pending U.S. patent applications Ser. Nos. 07/653,012, 07/807,062, 07/871,282, 07/886,752, and 07/893,981 which are hereby incorporated by reference.
There remains a need for small (to enhance blood and background tissue clearance), synthetic (to make routine manufacture practicable and to ease regulatory acceptance), high-affinity, specific-binding molecules radiolabeled with a convenient radiolabel, preferably Tc-99m, for use in imaging thrombi in vivo. Small synthetic compounds that bind specifically to the GPIlb/IIIa receptor on activated platelets, that are radiolabeled with a conventient radioisotope, preferably Tc-99m, .sup.111 In or .sup.68 Ga, fulfill this need in the art, and are provided by this invention.